While you might think that voluntary means something which is “done without compulsion or obligation” or “done, made, brought about, undertaken, etc., of one’s own accord or by free choice,” when it comes to America’s “voluntary” tax system, it’s quite the opposite.
If you freely choose not to voluntarily participate in the tax system, you will enjoy being raided by armed agents, prosecuted and sent to federal prison. Quite the voluntary system indeed.
Furthermore, there are some practices that are clearly in no way voluntary. When income taxes are withheld from paychecks, for example, there is nothing voluntary about it.